When you mix two waves, there are two beats produced. The frequencies of the beats are (the sum) and (the difference) of the frequencies of the two original waves. So we have to find the frequencies of these two forks.
Frequency = (speed) / (wavelength)
Fork #1: Frequency = (344 m/s) / (0.337 m) = 1020.8 Hz
Fork #2: Frequency = (344 m/s) / (0.3127 m) = 1100.1 Hz
The two beat frequencies are 2120.9 Hz and 79.3 Hz.
From the choices allowed, the one they want you to pick is 80 Hz .
